Contents that should be protected from illegal copies or external leakage, such as copyrighted works and individual information, for example, are stored in a target device in an encrypted state. The stored encrypted contents can only be decrypted by a host system that has succeeded in an authentication process performed between the host system and the target device. This prevents unauthorized host systems from performing illegal copies, leakage and the like. The target device herein is a storage medium such as an SD card (memory card), for example, or a device including such a storage medium. And the host system herein is a semiconductor integrated circuit that reads or writes data from or into a target device, a set system including such a semiconductor integrated circuit, or a content delivery system that delivers contents to a target device.
To be specific, the decryption described above is performed as follows. The host system reads an encrypted content key stored together with encrypted contents in the target device and decrypts the encrypted content key using an intermediate key generated in an authentication process, for example. Next, with the content key obtained by the decryption, the encrypted contents are decrypted, whereby the contents in plaintext form are obtained by the two-step decryption process.
Also, in order to allow only specified users to use electronically delivered encrypted contents, for example, a domain key or the like may be used. Specifically, the host system decrypts an encrypted domain key using the intermediate key, decrypts the encrypted content key with the obtained domain key, and then decrypts the encrypted contents using the obtained content key, whereby the contents in plaintext form are obtained by the three-step decryption process.
However, if the decrypted domain key, content key, or the like can be obtained by observing the processes mentioned above, the confidentiality of the contents decreases significantly. A technique has been therefore known which enables encryption processes, decryption processes and the like to be performed only within a semiconductor integrated circuit so that a content key and the like used during the processes cannot be obtained from outside the semiconductor integrated circuit (see, for example, Patent Document 1).
Patent Document 1: Japanese Laid-Open Publication No. 2004-096666